Origin of suspended matter and sediment inferred from the residual metal fraction: Application to the Marennes Oleron Bay, France

Continental Shelf Research 72 (2014) 119–130

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Origin of suspended matter and sediment inferred from the residual metal fraction: Application to the Marennes Oleron Bay, France Aymeric Dabrin b,n, Jörg Schäfer a, Olivia Bertrand a, Matthieu Masson a, Gérard Blanc a a b

Univ. Bordeaux, EPOC, UMR 5805, F-33400 Talence, France Irstea, UR MALY, 5 rue de la Doua, CS 70077, 69626 Villeurbanne, France

art ic l e i nf o

a b s t r a c t

Article history: Received 20 June 2012 Received in revised form 17 July 2013 Accepted 22 July 2013 Available online 2 August 2013

Total and HCl-available (extracted by HCl 1 M) trace metal (V, Cu, As, Cd, Pb and Th) concentrations were measured in suspended particulate matter (SPM) and surface sediments from the Garonne and the Charente Rivers (Southwest France), from their respective estuaries and from the adjacent coastal zone including the Marennes Oleron Bay. The objectives were to explore the potential of trace element signatures in the residual (non-reactive) fraction to trace the origin of particles, i.e. major contaminant carriers, in complex coastal systems. The observation period covered 12 months and a wide range of hydrological conditions. Selective extractions (HCl 1 M) showed that Pb, Cd and Cu in SPM and surface sediments from the entire system, including freshwater and marine environments, were highly reactive with potentially available fractions representing 64 7 13%, 60 718% and 437 13%, of the respective total metal concentrations. In contrast, V, As and Th showed low reactivity, with potentially available fractions lower than 19 7 7%, 11 7 3%, and 1.67 0.9%, respectively. Two combinations of Th-normalised (i.e. grain size corrected) metal concentrations in the residual fraction (Vres+Asres)/Thres and (Cdres+Cures+Pbres)/Thres represented two distinct signatures corresponding to the Charente and Garonne River endmembers. The respective Mres signals showed that both SPM and surface sediments sampled in the Marennes Oleron Bay mainly (454%) originated from the Gironde Estuary watershed. Furthermore, SPM from the tidal ranges of the Charente Estuary contained close to 100% of particles from the Gironde watershed. These results, together with SPM flux estimates for the Charente River, suggest that particle transport from the Gironde Estuary to the Marennes Oleron Bay accounts for 81,000 t a
1–435,000 t a
1, representing 5–29% of the total SPM expulsed by the Gironde Estuary. Application of this novel approach to determine SPM and sediment origin to other aquatic environments may require adapting the choice and association of residual metal concentrations. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Geochemical signature Marennes Oleron Bay Gironde Estuary Charente Estuary Trace metal Residual fraction

1. Introduction Particle transport through estuaries is a major vector for nutrient and pollutant fluxes to the continental shelf zones representing essential areas for sea food production (Horowitz et al., 2001; Neal et al., 1997). The various physical, geochemical and biological processes during transport modify composition and properties of suspended particulate matter (SPM) and control reactivity and fate of the associated trace elements (Turner and Millward, 2000). Offshore mudflats and open lagoons often are sinks for SPM and accumulate trace metals (Salomons and Forstner, 1984) which may be recycled and released to the water column by early diagenesis, bioturbation,

n

Corresponding author. Tel.: +33 4 72 20 10 53; fax: +33 4 78 47 78 75. E-mail addresses: [email protected], [email protected] (A. Dabrin). 0278-4343/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2013.07.008

wave dynamics or anthropogenic activities (dredging), especially in shallow waters (Kalnejais et al., 2010). Trace metals may easily enter the trophic system, accumulate in living organisms and affect ecosystem health, inducing risks for regional economy and/or human health (Harada, 1995; Lanceleur et al., 2011). Accordingly, it is essential to obtain information on sources and fate of particles circulating in the coastal zones to develop efficient sediment management and control strategies. Coastal zones may receive the drainage of different estuaries (Sánchez-García et al., 2009) and consequently a large variety of SPM characterised by their own chemical and physical properties. The trace element composition of SPM exported to the coastal ocean reflects the (i) background composition of the corresponding watershed due to bedrock weathering, (ii) anthropogenic inputs and (iii) chemical and physical processes occurring during their transport. Many studies focused on SPM/sediment sources, based on source tracing or “fingerprinting” (Wall and Wilding, 1976;

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significance of source group variability and source group sampling density to the accuracy of model output. The authors also discussed the fact that additional confounding factors such as non-conservative tracer behaviour and enrichment/depletion during the sediment delivery process represent a main concern. In this way, relatively few studies addressed sediment origin in estuaries and coastal environments by applying multi-element signatures, probably due to evolution of non-conservative element signatures in the estuarine gradients (e.g. salinity, turbidity, and redox), due to intense biogeochemical processes, e.g. organic matter degradation, flocculation, desorption, salting out, or flocculation (Dabrin et al., 2009; Masson et al., 2011; Turner and Millward, 2000). Araújo et al. (2002) analysed the lithogenic elemental (Al, K, Rb and rare earth elements) composition in fine estuarine mud patch sediments on the northern Portuguese and on Douro River sediments and showed similarities between these two sets of sediments. Given the important accumulation of sediments of heterogeneous origin in coastal environment (Sánchez-García et al., 2009) and the importance of particle transport in the behaviour of associated contaminants, successful sediment management requires reliable tools to trace particle sources and fate. The Marennes Oleron Bay (MOB; Atlantic Ocean; southwest France, Fig. 1) is a macro-tidal lagoon hosting the most important oyster (Crassostrea gigas) farming area in Europe with an annual oyster production of 40,000 t. The MOB receives direct water and SPM inputs via the Charente River, but depending on hydrological conditions, the bay may also receive water masses and SPM from the historically metal-polluted Garonne-Gironde fluvial estuarine system, located  15 km south of the Maumusson Strait ((Audry et al., 2007; Boutier et al., 2000; Lanceleur et al., 2011; Strady et al., 2011b); Fig. 1).

Walling et al., 1979), involving the selection of physical and chemical properties to discriminate the different potential sources of SPM. The applied fingerprinting techniques used sediment colour (Peart, 1993) magnetic properties (Walling et al., 1979), organic matter composition (e.g. C/N ratio, loss on ignition; (Peart, 1993)) or radionuclides (Walling and Woodward, 1992) to assess sediment origin in freshwater systems. More recent studies successfully used major and trace element signatures of stream SPM, bed sediments and soils, showing the potential of multi-element signatures to identify sediment sources at the watershed scale (Schäfer and Blanc, 2002; Stutter et al., 2009). Sediment fingerprinting has been developed over the past three decades for watershed sediment transport research. This method allows allocation of sediment nonpoint source pollutants in a watershed through the use of natural tracer technology with a combination of field data collection, laboratory analyses of sediments, and statistical modelling techniques (Davis and Fox, 2009). However, there is no single diagnostic property capable of discriminating the range of potential suspended sediment sources in different basins (Collins and Walling, 2002). More specifically, these authors suggest that measurements of a combination of acid and pyrophosphate-dithionite extractable metals, base cations and organic constituents should provide an effective basis for establishing composite fingerprints for discriminating individual sediment source types. Radiometric properties also provide useful information for improving sediment source discrimination. Moreover, there is a need to refine existing approaches to take account of a variety of sources of uncertainty (Collins et al., 2010). Thus, Small et al. (2004) applied a Bayesian approach to the multivariate problem of “unmixing” sediment sources, showing the

RéIsland Antioche Strait

Marennes Oleron Bay

Oléron Island

N Charente

Maumusson Strait WGMP

La Cayenne

France

Chaniers

Ronce les Bains

Bay of Biscay Seudre

Gironde Estuary

Spain Isle

SGMP

Dordogne River Bordeaux Garonne River La Réole 20 km

Fig. 1. Map of the study area and sampling stations. Full squares at the La Réole and Chaniers sites correspond respectively to the long-term sampling sites on the Garonne and Charente Rivers. Open and full circles represent sampling sites in the Gironde Estuary during the GIMERCAD cruise (March 2007) and the METOGIR cruise (July 2005). Open squares correspond to sampling sites in the Charente Estuary during the CHAMET cruise (November 2006). Dotted areas symbolise the fine-grained deposits offshore the Gironde mouth (West Gironde Mud Patch: WGMP; South Gironde Mud Patch: SGMP). Black stars represent the surface sediment sampling locations. The two crosses show the two sampling sites in the South part of the Marennes Oleron Bay (La Cayenne and Ronce les Bains sites).

A. Dabrin et al. / Continental Shelf Research 72 (2014) 119–130

This implies that metal mobilisation from Gironde SPM may occur inside the MOB and that these additional metal inputs may represent a risk to this vulnerable oyster production zone. The objectives of this study were to (i) determine and compare total trace metal concentrations and HCl-available fractions in SPM from the Charente and Garonne Rivers, (ii) evaluate the potential of the (non-reactive) residual metal fractions of the Charente and Gironde SPM as geochemical signatures allowing to determine SPM origin in coastal zones, and (iii) apply this new geochemical approach to estimate the contribution of the Gironde SPM inputs to the annual SPM budget of the Marennes Oleron Bay.

2. Material and methods 2.1. Study area The Marennes Oleron Bay (MOB; 190 km2; French Atlantic Ocean coastline) is a macro-tidal bay delimited by the Oleron Island to the West and the Charente shoreline to the East (Fig. 1). The bay receives direct inputs of freshwater and SPM by two tributaries: the Seudre River (watershed area  400 km2; average daily freshwater discharge o40 m3 s
1) and the Charente River (watershed area 10,000 km2; average daily freshwater discharge 10–600 m3 s
1). Water circulation inside the MOB mainly occurs along a central channel separating two large intertidal mudflats which represent 60% of the total area of the embayment (Bassoullet et al., 2000). Sediment dynamics and intertidal mudflat characteristics have been well defined in several studies (Bassoullet et al., 2000; Gouleau et al., 2000). Oyster production is particularly intense in the southeastern part of MOB, near the Maumusson Strait, which has occasionally hydraulic connection

121

with the Gironde Estuary water masses, depending on hydrology and wind directions (Dabrin, 2009). The physical, geochemical and hydrological characteristics of the Gironde Estuary (  170 km length,  80,000 km2 watershed surface area; Fig. 1) have been well defined in several studies (Schäfer et al., 2002; Sottolichio and Castaing, 1999). The Gironde Estuary receives mean annual freshwater discharge of  1000 m3 s
1 via the Dordogne and Garonne Rivers and  3  106 t a
1 of fluvial SPM (1990–1999 period), the Garonne River contributing  74% of the total SPM inputs (Schäfer et al., 2002). The maximum turbidity zone (MTZ) is typically located in the low salinity region and migrates to the estuary mouth during major flood events, resulting in significant SPM explusion to the coastal ocean (Sottolichio and Castaing, 1999). Fifteen kilometres offshore the Gironde Estuary mouth, the West Gironde Mud Patch (WGMP; 420 km2 surface area), a zone of fine-grained sediments, traps  30% of the total SPM exported by the Gironde Estuary corresponding to an annual deposit of about 3.6  105 t (Jouanneau et al., 1989; Parra et al., 1999). 2.2. Sampling The sampling strategy aimed at covering the sites and periods representative of the main particle transport in the Gironde and the Charente fluvial-estuarine systems and in the MOB (Fig. 1; Tables 1–3). River SPM samples were collected at the La Réole and Chaniers Sites on the Garonne and the Charente Rivers, respectively (Fig. 1). Both sites represent the major entries of the respective estuaries, i.e. they integrate the most important watershed areas of their respective watersheds and are located in the freshwater reaches

Table 1 Hydrological conditions and SPM concentrations during sampling campaigns on the Garonne and Charente Rivers. Total and HCl-extracted metal concentrations are expressed in mg kg
1 of dry weight. Discharge (m3 s
1)

Garonne River 8 February 2001 5 April 2001 25 April 2001 17 May 2001 21 June 2001 18 July 2001 22 August 2001 26 September 2001 25 October 2001 21 November 2001 6 February 2003 23 November 2003

1016 878 929 1012 388 287 117 153 324 140 4196 810

SPM (mg L
1)

14 22 51 153 6 27 7 4 24 4 653 117

Mean sd Charente River 31 August 2005 19 April 2006 19 June 2006 31 August 2006 25 September 2006 23 October 2006 25 January 2007 23 February 2007 27 March 2007 11 May 2007 4 June 2007 Mean sd

o 10 84 19 14 40 58 166 171 117 40 57

5 8 7 6 3 4 31 8 6 4 5

V (mg kg
1)

Cu (mg kg
1)

As (mg kg
1)

Cd (mg kg
1)

Pb (mg kg
1)

Th (mg kg
1)

Total

HCl 1 M

Total

HCl 1 M

Total

HCl 1 M

Total

HCl 1 M

Total

HCl 1 M

Total

HCl 1 M

97.6 112 98.2 172 134 138 117 114 122 126 135 96.1

7.80 8.90 7.10 12.4 11.2 10.8 8.96 9.52 9.23 10.5 9.80 8.13

36.7 29.8 37.8 34.2 31.0 32.0 38.7 44.4 35.9 38.1 53.6 50.3

15.3 17.4 20.6 13.1 14.6 16.0 27.1 33.9 22.3 21.4 10.9 30.6

18.2 31.1 20.9 24.1 23.4 25.0 19.8 22.3 24.5 29.7 29.4 46.3

2.30 3.73 2.80 3.20 3.00 0.40 3.60 5.00 3.10 3.56 3.67 10.9

5.34 4.30 1.35 1.27 1.74 1.17 1.42 1.66 1.95 2.71 1.24 2.85

5.11 4.13 1.34 1.13 1.62 1.02 1.28 1.53 1.81 2.49 0.87 2.34

70.0 65.9 34.3 35.6 69.3 42.0 66.9 50.3 61.2 68.4 53.1 128

65.3 50.8 32.1 30.0 40.6 38.9 39.9 41.1 46.5 49.7 34.2 98.5

9.84 14.7 13.2 8.40 16.7 20.3 17.9 18.1 18.8 19.2 13.0 12.7

0.22 0.24 0.21 0.56 0.36 0.26 0.21 0.20 0.66 0.30 0.09 0.15

122 21.5

9.53 1.52

38.5 7.42

20.3 7.18

26.2 7.48

3.77 2.49

2.25 1.34

2.06 1.31

62.1 24.6

47.3 18.8

15.2 3.85

0.29 0.17

51.3 43.5 63.5 32.8 41.8 32.0 54.9 59.0 51.7 31.0 35.4

5.90 4.73 5.57 6.23 4.74 4.43 6.00 6.82 5.59 3.82 3.96

69.2 83.1 161 66.1 116 91.3 53.9 142 64.5 37.9 85.8

30.4 42.8 102 47.2 50.8 45.8 25.5 94.9 29.8 17 33.2

10.1 9.89 11.5 8.95 8.62 7.49 11.5 15.3 10.7 10.0 18.7

2.50 3.01 2.88 4.72 2.53 2.59 2.87 2.75 2.86 2.21 8.34

2.25 5.24 3.94 6.75 5.19 7.11 3.77 4.67 4.77 1.75 24.0

1.40 2.80 2.42 5.21 2.91 3.79 2.20 2.93 2.77 0.99 14.1

90.6 107 98.3 94.2 115 116 67.0 126 77.9 50.5 216

38.3 54.4 54.9 73.5 57.9 66.7 34.7 71.6 38.7 26.7 168

6.01 5.29 9.42 3.92 6.58 2.82 5.90 6.23 5.48 3.94 4.70

0.06 0.06 0.05 0.06 0.05 0.05 0.07 0.07 0.05 0.04 0.05

45.2 11.6

5.25 0.98

88.3 37.6

47.2 27.3

11.2 3.21

3.39 1.77

6.31 6.09

3.77 3.60

105 43.0

62.3 38.3

5.48 1.74

0.06 0.01

122

A. Dabrin et al. / Continental Shelf Research 72 (2014) 119–130

Table 2 Hydrological conditions and SPM concentrations during sampling campaigns on the Gironde and Charente Estuaries. Total and HCl-extracted trace metals concentrations in SPM are expressed in mg kg
1 of dry weight. Sample location

Discharge (m3 s
1)

Gironde Estuary MTG 101 July MTG 103 July MTG 104 July MTG 105 July MTG 106 July MTG 107 July MTG 109 July MTG 110 July MTG 113 July MTG 114 July GMC GMC GMC GMC GMC GMC GMC GMC GMC GMC GMC

301 303 305 307 309 311 312 313 315 317 323

2005 2005 2005 2005 2005 2005 2005 2005 2005 2005

March March March March March March March March March March March

2007 2007 2007 2007 2007 2007 2007 2007 2007 2007 2007

Salinity SPM (mg L
1)

V (mg kg
1)

Cu (mg kg
1) As (mg kg
1)

Cd (mg kg
1) Pb (mg kg
1)

Th (mg kg
1)

Total HCl 1M

Total HCl 1M

Total HCl 1M

Total HCl 1M

Total HCl 1M

Total HCl 1M

304 304 304 304 304 304 304 304 304 304

0.7 2.3 2.5 4.0 5.7 7.6 11.5 13.9 18.8 21.0

938 234 141 405 242 524 79 47 81 172

138 147 148 141 145 139 141 143 140 123

11.5 10.6 10.5 13.0 12.3 12.9 11.5 12.6 12.9 11.8

24.7 25.9 26.7 25.3 30.9 31.5 34.1 34.8 34.0 30.5

10.8 10.7 10.3 12.1 13.3 12.3 11.8 16.0 14.0 12.9

23.2 23.3 23.5 25.5 25.7 27.5 26.6 27.5 28.8 26.6

3.56 3.22 3.37 4.22 4.07 4.27 3.53 3.59 3.81 4.04

0.43 0.47 0.58 0.53 0.67 0.53 0.86 0.70 0.62 0.54

0.26 0.27 0.38 0.30 0.44 0.28 0.48 0.47 0.34 0.30

54.2 49.0 52.9 61.1 61.7 68.1 64.7 66.3 75.0 66.6

33.7 28.6 31.1 40.0 38.4 40.0 37.2 39.5 43.0 39.4

12.7 12.8 13.1 14.3 14.2 13.9 14.7 15.1 15.6 14.1

0.22 0.19 0.19 0.24 0.21 0.22 0.17 0.19 0.16 0.19

2600 2600 2600 2600 2600 2600 2600 2600 2600 2600 2600

0 0 0 0.8 3.1 4.6 4.8 6.7 9.0 11 19

65 334 297 979 1645 625 32200 284 259 491 115

103 98.4 94.9 117 119 111 123 124 129 125 125

10.7 11.1 12.2 12.7 12.9 10.5 11.8 13.4 13.4 18.8 15.3

40.9 35.2 27.4 29.1 29.1 28.2 34.5 33.6 36.3 33.2 33.7

23.1 17.2 11.9 12.4 11.8 9.87 10.8 12.4 13.5 16.0 16.3

23.6 21.0 20.0 23.1 23.2 21.0 27.1 26.1 26.0 26.0 24.8

4.25 3.74 3.84 4.26 4.04 3.58 3.91 4.06 3.90 5.35 4.46

0.74 0.61 0.54 0.47 0.57 0.44 0.51 0.58 0.53 0.51 0.4

0.51 0.37 0.34 0.27 0.35 0.24 0.27 0.33 0.30 0.34 0.23

57.0 50.8 46.8 50.1 52.1 49.3 63.6 61.0 61.3 61.2 56.8

42.4 35.8 32.7 38.5 40.6 32.2 32.9 41.1 40.7 54.3 43.9

12.0 11.3 10.4 8.72 9.95 10.6 12.0 12.4 12.4 12.5 11.7

0.20 0.25 0.30 0.24 0.24 0.22 0.24 0.23 0.24 0.35 0.24

127 16.0

12.5 1.87

31.4 4.18

13.3 3.05

24.8 2.38

3.96 0.46

0.56 0.11

0.34 0.08

58.6 7.52

38.4 5.59

12.6 1.78

0.23 0.04

121 124 126 126 125 126 130 126 126 124

13.9 14.5 14.9 14.3 14.1 14.1 15.3 14.9 14.1 14.2

20.3 22.4 20.9 20.0 22.1 20.5 21.4 22.7 21.2 25.2

4.57 4.63 4.95 4.59 4.70 4.69 5.03 4.95 4.65 4.74

19.9 20.6 19.5 20.5 21.2 21.4 21.5 21.7 22.2 22.1

3.87 3.59 4.32 3.65 3.64 3.60 3.93 3.88 3.95 3.58

0.44 0.52 0.35 0.34 0.49 0.37 0.36 0.51 0.36 0.66

0.12 0.12 0.11 0.14 0.13 0.14 0.15 0.14 0.14 0.14

53.9 55.8 53.1 55.8 58.9 55.3 54.6 59.2 49.7 62.4

24.4 26.6 26.7 26.4 25.7 25.4 28.6 28.4 25.5 27.7

12.1 12.0 12.3 12.6 12.8 12.7 12.7 12.8 11.9 12.3

0.14 0.16 0.16 0.15 0.14 0.15 0.18 0.18 0.16 0.17

125 2.27

14.4 0.46

21.7 1.53

4.75 0.17

21.1 0.91

3.80 0.24

0.44 0.10

0.13 0.01

55.9 3.57

26.5 1.36

12.4 0.34

0.16 0.01

Mean sd Charente Estuary CHAMET24 CHAMET25 CHAMET31 CHAMET32 CHAMET33 CHAMET34 CHAMET36 CHAMET38 CHAMET39 CHAMET40 Mean sd

October October October October October October October October October October

2006 2006 2006 2006 2006 2006 2006 2006 2006 2006

60 60 60 60 60 60 60 60 60 60

10.5 8.5 5.5 6.5 4.2 4.0 3.4 2.3 1.5 1.4

1044 774 533 1063 1664 2937 1175 2905 579 2453

upstream of the limit of the dynamic tide. The SPM samples studied cover different hydrological conditions. The Garonne River SPM (n ¼10) were collected between February and November 2001 (Audry et al., 2006), and during two major flood events in 2003. A sample was collected from the Charente River during pronounced low water discharge in August 2005 and during one hydrological year (April 2006–June 2007; n¼ 10). Gironde Estuary SPM were collected onboard the research vessel Côtes de la Manche (INSU) during two cruises covering contrasting hydrological conditions (Table 2), i.e. low freshwater discharge (304 m3 s
1; METOGIR cruise; n ¼ 10; July 2005) and flood (2600 m3 s
1; GIMERCAD 3 cruise; n ¼11; March 2007). Additionally, estuarine SPM samples were collected in the northern and central part of the MOB (n ¼7; METOGIR cruise) and in the Charente Estuary (n¼ 10; CHAMET cruise) during midlevel freshwater discharge (60 m3/s). Two surface sediment samples were sampled with a Shipek grab from the WGMP and from the Antioche Strait (Fig. 1; Table 3). In September 2006, a sediment sample from intertidal mudflats was collected on the left bank of the Gironde Estuary. Finally, SPM were collected between January and October 2007 in the southern part of the MOB, at the La Cayenne (n ¼ 6) and Ronce-les-Bains (n ¼6) sites (Fig. 1; Table 3).

On the river/estuary banks and onboard the research vessel, SPM were retrieved by pumping of up to 1000 L of water using peristaltic pump with polypropylene PP tubes followed by centrifugation (Wesfalia separator, 12,000 g). This method provides quantitative SPM recovery, without changing the chemical composition of the particulate matter (Schäfer and Blanc, 2002). Sediments and SPM were systematically dried at 50 1C, homogenised and stored in PP tubes in a dry atmosphere and in the dark in order to keep the chemical integrity of samples, allowing to measure metal concentration after long period of storage (NF EN ISO 5667-15, 2009). Determination of SPM concentrations was performed by filtration of precise raw water volumes through dry, pre-weighed filters (Whatman GF/F, 0.7 mm), followed by a drying and weighing step. 2.3. Particulate ETM extraction and analysis Total metal concentrations in SPM and sediments were analysed from representative sub-samples (30 mg of dry, powdered and homogenised material) digested in closed, previously acid-cleaned PP reactors (DigiTUBEs; SCP Sciences) using 1.5 ml HCl (12 M, Suprapur), 0.5 ml HNO3 (14 M, Suprapur) and 2 ml HF (22 M,

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Table 3 Total and HCl-extracted trace metals concentrations in SPM from the Marennes Oleron Bay and in three sediments of the coastal system (expressed in mg kg
1 of dry weight). Sample location

Salinity

SPM (mg L
1)

V (mg kg
1)

Cu (mg kg
1)

As (mg kg
1)

Cd (mg kg
1)

Pb (mg kg
1)

Th (mg kg
1)

Total

HCl 1 M

Total

HCl 1 M

Total

HCl 1 M

Total

HCl 1 M

Total

HCl 1 M

Total

HCl 1 M

Marennes Oleron Bay MTG130 MTG131 MTG133 MTG134 MTG135 MTG136 MTG143

33.9 34.1 34.2 34.3 34.3 34.4 34.3

20 15 10 8 13 15 3

121 101 71.3 45.1 81.1 100 38.1

13.3 11.0 7.22 5.15 8.1 9.94 4.63

28.7 34.1 24.4 26.9 28.3 39.8 29.2

10.3 15.7 7.07 8.69 8.64 12.3 11.4

20.5 17.0 11.6 8.3 12.4 17.2 6.51

3.17 0.61 2.04 1.77 2.29 3.11 1.67

0.26 0.27 0.17 0.17 0.29 0.27 0.09

0.16 0.13 0.06 0.07 0.12 0.10 0.05

51.3 40.2 24.6 13.8 39.8 39.4 14.7

28.2 21.4 14.5 8.78 20.1 19.6 11.2

11.6 9.37 6.83 4.25 7.85 8.79 3.72

0.11 0.09 0.05 0.02 0.06 0.08 0.02

La Cayenne January 2007 February 2007 March 2007 April 2007 June 2007 October 2007

31.4 29.8 20.1 31.5 31.9 35.0

190 34 39 112 13 121

122 121 111 141 117 113

15.8 22.0 16.0 17.3 13.1 15.8

30.5 45.7 35.0 28.8 49.9 32.5

12.1 30.4 18.4 10.5 23.4 13.9

21.7 24.5 20.8 25.8 21.5 20.7

4.21 6.89 5.06 4.88 3.76 4.69

0.77 1.63 0.90 0.51 0.86 0.79

0.44 1.35 0.5 0.27 0.39 0.51

63.6 68.9 60.2 60.5 93.0 62.8

37.8 59.5 44.0 33.7 63.2 44.0

10.9 10.7 10.4 11.8 9.30 10.5

0.14 0.19 0.15 0.13 0.11 0.15

Ronce les Bains January 2007 February 2007 March 2007 April 2007 June 2007 October 2007

31.7 30.9 23.8 31.7 31.5 35.1

53 35 38 37 26 55

134 123 105 122 84.1 113

16.6 25.1 15.5 16.1 10.2 17.0

45.0 44.6 31.3 52.9 40.5 31.2

21.0 31.6 15.4 22.8 17.3 12.7

22.1 24.1 19.5 22.3 15.9 21.0

4.20 6.77 4.59 4.00 3.55 5.17

1.11 1.09 0.35 0.76 0.99 0.53

0.64 1.01 0.21 0.35 0.52 0.33

63.5 63.8 55.7 73.3 99.7 53.7

40 60.6 40.4 45.8 64.2 38.4

11.4 10.7 9.34 10.5 6.61 10.5

0.15 0.20 0.14 0.11 0.06 0.14

103 27.9

13.7 5.36

35.8 8.44

16.0 7.07

18.6 5.44

3.81 1.67

0.62 0.41

0.38 0.34

54.9 22.7

36.6 17.6

9.21 2.36

0.11 0.05

111 90.2 137

14.6 19.0 23.2

29.8 14.3 17.9

14.0 3.78 6.12

20.6 15.3 29.3

3.32 3.28 5.65

0.63 0.18 0.18

0.43 0.10 0.07

53.7 39.6 31.0

42.3 30.9 21.6

11.7 8.50 11.6

0.32 0.23 0.50

Mean sd Gironde sediment West Gironde Mud Patch Antioche Straits

– – –

Sed Sed Sed

Suprapur). The reactors were then kept at 110 1C in an automatic heating block (DigiPREP, SCP Sciences) for 2 h and after complete cooling the digested solutions were evaporated to dryness. Extraction of acid-soluble metals (HCl 1 M) is commonly applied to quantify the metal fractions bound to the reactive phases (Audry et al., 2006; Morse and Luther, 1999), i.e. the potentially bioavailable metal fraction. The non-reactive (residual) metal fraction is commonly considered as not bioavailable.The HCl-available fraction was extracted by HCl (1 M, Suprapur; 12.5 ml) from 200 mg of homogenised material, continuously agitated for 24 h in previously acidcleaned PP centrifuge tubes (50 ml; Greiner bio-one) as described elsewhere (Audry et al., 2006). The samples were then centrifuged during 30 min at 4000 rpm. Aliquots of the supernatant were transferred into acid-cleaned PP reactors (DigiTUBEs; SCP Sciences) reactors and evaporated to dryness at 100 1C. The dry residues of both total digestions and HCl-extractions were dissolved with 250 ml HNO3 (14 M, Suprapur) and 5 ml of double deionized water (Milli-Qs) followed by heating for 30 min at 100 1C. Exactly 3.5 ml of the solution was brought to 10 ml using 6.5 ml of double deionized water (Milli-Qs) and stored at 4 1C in the dark pending ICP-MS analysis (Thermo, X7) with external calibration under standard conditions. Sixteen trace elements were analysed (data not shown) but according to the objective of the present work, only V, As, Pb, Cd, Cu and Th results were reported, since they allowed discrimination of the signature of the particles. For each extraction/digestion batch, blanks and international certified reference materials (estuarine sediment; IAEA 405) were processed in parallel, i.e. in the same conditions. Method blank values were subtracted, reproducibility for both total digestions and HCl extractions was generally better than 15% rsd, and element-dependent recoveries estimated from total digestions of reference sediments were 84–114% for V, As, Pb, Cd, Cu and Th (Table 4).

2.4. SPM fluxes in the Garonne, Charente and Seudre Rivers Annual SPM fluxes at the Chaniers Site on the Charente River (2006 and 2007) and at the La Réole site on the Garonne River were derived from our permanent monitoring network (Fig. 1), using daily measured discharge and SPM concentrations and commonly applied algorithms (Horowitz et al., 2001; Masson et al., 2006). The freshwater discharges for the Garonne, Charente and Seudre Rivers were provided by national and regional offices. At the Chaniers site, daily cumulative samples (1 L) of river water consisting of eight subsamples taken at regular time intervals (180 min) were sampled at 1 m from the riverbank at 0.5 m depth, by automatic sampling systems (Sigma 900P). At the La Réole site, daily 1 L grab samples were retrieved at 1 m from the riverbank at  0.5 m depth. Filtration of precise volumes through preweighed glass fibre filters (Durrieu, 0.7 mm), followed by drying and reweighing provided representative daily SPM concentrations.

3. Results 3.1. Hydrological conditions, SPM concentrations and fluxes The SPM collected on the Garonne and Charente Rivers were representative of contrasting hydrological conditions as occurring during typical hydrological years (Table 1). In the studied rivers, freshwater discharges were at minimum in summer and at maximum in winter (Table 1), i.e. consistent with the typical interannual hydrological cycles in the system (Masson et al., 2007). The observed SPM concentrations were relatively high and variable (4–653 mg L
1 in the Garonne River), while they were clearly lower and less variable in the Charente River (typically 3–8 mg L
1, with one exceptional value of 31 mg L
1; Table 1).

124

A. Dabrin et al. / Continental Shelf Research 72 (2014) 119–130

Table 4 Trace metal concentrations of the certified reference material IAEA 405, interval of confidence and the measured concentrations with accuracy and reproducibility. n ¼10

As

Cd

Cu

Th

V

IC 95% Certified values (mg kg
1) Measured values (mg kg
1) Accuracy (%) Reproducibility (%)

22.9–24.3 23.6 24.6 104 9

0.68–0.78 0.73 0.85 114 15

46.5–48.9 47.7 48 101 7

12.2–16.4 14.3 12.4 84 5

90–100 95 93.5 98 4

Based on daily discharge and SPM concentrations, we estimated the annual SPM fluxes (2006–2007 period) at  76,000 t a
1 for the Charente River. The respective fluxes in the Garonne River were 0.90 Mt in 2006 and 0.87 Mt in 2007. The SPM concentrations along the salinity gradient in the Gironde Estuary displayed very different levels and distribution patterns during the two sampling cruises, i.e. in contrasting hydrological conditions (Table 2). In July (METOGIR), maximum SPM concentrations (938 mg L
1) occurred at the onset of the salinity gradient indicating the upstream position of the MTZ. Then, SPM concentrations decreased with the increasing salinity to a minimum of 47 mg L
1 at S¼ 14, i.e. in the mid-salinity range followed by a moderate increase towards the estuary mouth (Table 2). In March (GIMERCAD 3), i.e. during the flood SPM, concentrations along the salinity gradient were clearly higher and reached 32,200 mg L
1 at S¼5. This and the clear shift of the salinity gradient towards the ocean (Table 2) are typical flood features in this hyper-turbid estuary (Dabrin et al., 2009). In the Charente Estuary (CHAMET), the highest SPM concentrations occurred in the low salinity range (1oSo10), ranging from 580 mg L
1 to 2940 mg L
1 corresponding to a MTZ location typical of macro-tidal estuaries. In the northern MOB, we observed salinities close to S¼ 34 and SPM concentrations were lower than 20 mg L
1 in July 2005 (Table 3), i.e. conditions similar to those in the adjacent coastal ocean (Strady et al., 2011a). In contrast, SPM in the southern MOB (La Cayenne and Ronce les Bains sites) were systematically higher, ranging from 26 mg L
1 up to 190 mg L
1 at the La Cayenne site and reaching 112 mg L
1 at the Ronces les Bains site in January 2007. Furthermore, the southern MOB waters typically had lower salinity (average S¼30.4) than those in the northern MOB (Table 3). 3.2. Total particulate trace metal concentrations Total particulate V, As and Th concentrations in SPM from the Charente and Garonne Rivers were relatively constant, whatever the water discharge (Table 1). Average concentrations of Vtot (122721 mg kg
1), Astot (26.277.5 mg kg
1) and Thtot (15.27 3.9 mg kg
1) in Garonne River SPM were respectively 2.6, 2.3 and 2.8 times higher than those in Charente River SPM (Vtot: 45.2 711.6 mg kg
1; Astot: 11.273.2 mg kg
1; and Thtot: 5.48 71.7 mg kg
1). Conversely, mean concentrations of Cutot (88.3 737.6 mg kg
1), Cdtot (6.317 6.09 mg kg
1) and Pbtot (105 743.0 mg kg
1) in Charente River SPM were respectively 2.3, 2.8 and 1.7 times higher than those in Garonne River SPM (Cutot: 38.5 77.4 mg kg
1; Cdtot: 2.25 71.3 mg kg
1; and Pbtot: 62.1 724.6 mg kg
1). In the Gironde Estuary, average metal concentrations during both cruises were 127 716 mg kg
1 for Vtot, 31.47 4.18 mg kg
1 for Cutot, 24.8 72.38 mg kg
1 for Astot, 0.56 7 0.11 mg kg
1 for Cdtot, 58.6 77.52 mg kg
1 for Pbtot and 12.6 7 1.78 mg kg
1 for Thtot (Table 2). The concentrations were relatively homogeneous (considering that the campaigns covered the whole salinity gradient and contrasting hydrological conditions), with a relative standard deviation (RSD) below 20%. Total particulate metal concentrations in SPM from Charente Estuary were close to those in the Gironde Estuary and showed low variability along the salinity gradient (Table 2), with a mean of

125 72.27 mg kg
1 for Vtot, 21.7 71.53 mg kg
1 for Cutot, 21.1 70.91 mg kg
1 for Astot, 0.447 0.10 mg kg
1 for Cdtot, 55.9 7 3.57 mg kg
1 for Pbtot, and 12.4 70.34 mg kg
1 for Thtot. Particulate trace metal concentrations in SPM from the northern MOB were generally lower than those in the Charente and Gironde Rivers/Estuaries and were more variable with RSD up to 48% (Table 3). Particulate metal concentrations in surface sediments from the Gironde Estuary were similar to those in the Gironde SPM and consistent to those reported in previous work on the spatial distribution of metal in Gironde Estuary in surface sediments (Larrose et al., 2010). Sediments from the WGMP and the Antioche Strait, however, showed clearly lower values (Table 3). 3.3. HCl-extracted metal concentrations The HCl-extracted metal fractions in SPM from the Garonne and Charente Rivers were highly variable, depending on the metal considered. In fact, the VHCl, AsHCl and ThHCl fractions represented less than 30% of the respective total metal concentrations, while CuHCl, CdHCl and PbHCl typically represented more than 53% (Table 1). Estuarine SPM samples from both the Gironde and the Charente Estuaries generally had lower metalHCl fractions than the respective river SPM, except for AsHCl and ThHCl, which were similar in estuary and river SPM (Tables 1 and 2). Inside the MOB, metalHCl fractions in SPM were higher in the southern part (La Cayenne and Ronce les Bains sites) than in the northern part. MetalHCl fractions in SPM and surface sediment from the Gironde Estuary had similar values. The VHCl, AsHCl, PbHCl and ThHCl concentrations in WGMP surface were similar to those observed in Gironde Estuary SPM, whereas CuHCl and CdHCl were clearly lower. The highest VHCl, AsHCl and ThHCl concentrations occurred in sediments from the Antioche Strait (Table 3).

4. Discussion 4.1. Temporal and spatial comparison of total particulate metal concentrations in river SPM The downstream Riou Mort-Lot-Garonne River system is known for historical metal pollution (e.g. Cd, Zn, Pb, Cu, and Hg), (Audry et al., 2004a, 2004b; Schäfer et al., 2002). However, the direct comparison of Cutot, Cdtot and Pbtot concentrations in Garonne and Charente River SPM showed that, against our expectations, the latter had clearly higher metal levels (Fig. 2a). To our knowledge this is the first comparison of metal concentrations in SPM from both systems covering entire hydrological years. In contrast to the well-studied Garonne River, only two previous studies focused on metals in Charente River SPM. Boutier et al. (2000) measured Cdtot concentrations at the Taillebourg site ( 20 km downstream from the sampling site in the present work) in 1991 and reported mean Cdtot values (5.28 mg kg
1) similar to our recent results (Fig. 2; Table 1). More recently, Schäfer and Blanc (2002) reported Cutot, Cdtot and Pbtot concentrations in Charente River SPM for normal flow and for a flood event in 1999. These Cutot and Cdtot values were relatively similar to our

A. Dabrin et al. / Continental Shelf Research 72 (2014) 119–130

160 140

mg kg-1

120 100 80 60 40 20 0

V

Cu

As

Cd

Pb

Th

25

Garonne River

ETM/Th

20

Charente River

125

Li (Loring, 1990), Sc (Audry et al., 2004c) or Cs (Ackermann, 1980). More recently, Th-normalisation has provided efficient correction of the grain size effect (Coynel et al., 2007). Thorium-normalised V and As concentrations showed similar ranges in the Garonne and Charente Rivers, suggesting similar grain-size corrected levels for both watersheds (Fig. 2). In contrast, Th-normalisation increased the differences observed for Cutot, Cdtot and Pbtot, with Cu/Th, Cd/Th and Pb/Th values up to 6-fold higher in Charente River SPM. These clear differences could be attributed to ore deposits in the upper Charente watershed (BRGM, 1979) and/or anthropogenic sources (industry, urban sources, etc.) and suggest that the Charente River may significantly contribute to trace metal inputs into the Marennes Oleron Bay. This is in contradiction to the existing paradigm, saying that the Gironde Estuary was the highly dominant metal source for the MOB (Boutier et al., 2000). Therefore, additional effort is necessary to identify and assess metal sources in the Charente watershed to support the definition of regional environmental management priorities, especially with respect to decreasing metal transport via the Gironde Estuary (Dabrin et al., 2009).

15

4.2. Total particulate metal concentrations in estuarine SPM 10

5

0

V/Th

Cu/Th

As/Th

Cd*10/ Th

Pb/ Th

Fig. 2. Average total (a) and Th-normalised metal concentrations (b) in SPM from the Garonne and the Charente Rivers.

recent results covering contrasting hydrological situations, suggesting that average Cutot and Cdtot levels have not changed during the last decade. The Pbtot values measured in Charente River SPM during one hydrological year were generally higher but close to those reported for the 1999 flood (Pbtot ¼87 mg kg
1, (Schäfer and Blanc, 2002)), but clearly lower than the respective 1999 normal flow value (Pbtot ¼ 468 mg kg
1). This result may reflect reduced Pb emissions in the watershed due to changing industrial pressure, remediation work in former mining areas, and/or the use of unleaded fuel as observed in other aquatic systems (ElbazPoulichet et al., 2011). The observed Vtot and Astot levels in Charente River SPM were consistent with previous data, while Thtot tended to be lower (Schäfer and Blanc, 2002). The latter could be due to the different sampling sites used for both studies, eventually inducing differences in SPM composition. In fact, the Taillebourg site is located close to, but downstream from the extreme upper limit of tidal influence, and may episodically be influenced by tidal currents. The resulting remobilisation and upstream transport of estuarine bottom sediment (Th-rich lithogenic particles; Van Calsteren and Thomas, 2006) could alter the SPM composition at the Taillebourg site. Accordingly, the low SPM concentrations and Thtot at our sampling site suggest that the lithogenic fraction is low and that a large proportion consists of particulate organic matter such as phytoplankton (Caetano et al., 2006). Particulate metal concentrations in sediment and SPM generally vary with grain size (Loring, 1990). Accordingly, comparing chemical compositions of SPM or sediment particles sampled in different hydrological conditions or in different aquatic systems commonly relies on normalisation, i.e. mathematical correction of the analytical results accounting for eventual biases (Summers et al., 1996). Major and minor mainly lithogenic elements have been used for normalisation like Fe (Tam and Yao, 1998), Al (Martin and Whitfield, 1983), Rb (Grant and Middleton, 1990),

Total particulate trace metal concentrations in SPM from both the Gironde and Charente Estuaries (Table 2) were rather constant along their respective salinity and turbidity gradients reflecting the fact that particles are well-mixed and have long average residence times (e.g. 41 year for the Gironde Estuary; (Jouanneau and Latouche, 1981). Although estuarine particle residence time depends on freshwater discharge, the Gironde SPM had similar metal concentrations during both low discharge and flood (Vtot: 127 716 mg kg
1, Cutot: 31.47 4.2 mg kg
1, Astot: 24.7 72.4 mg kg
1, Cdtot: 0.44 70.1 mg kg
1, Pbtot: 58.5 7 7.5 mg kg
1, and Thtot: 12.6 71.8 mg kg
1). Furthermore, recent Cdtot, Cutot and Pbtot concentrations in the Gironde Estuary SPM were close to those observed in 1994 (Kraepiel et al., 1997), Cdtot: 0.54 mg kg
1, Cutot: 36.5 mg kg
1 and Pbtot: 56.2 mg kg
1) although metal gross fluxes into the estuary have clearly decreased during the last decade (Dabrin et al., 2009; Masson et al., 2006). Although there is much less data for the Charente Estuary, Cdtot concentrations within the Charente Estuary salinity gradient reported for 1991 (Boutier et al., 2000) were similar to our recent results (Table 2). This suggests that (i) the chemical composition of estuarine particles expulsed to the coastal ocean does not change rapidly, at least in huge, macrotidal estuaries and (ii) total metal concentrations in estuarine SPM mainly reflect particle composition modified by biogeochemical processes (e.g. organic matter degradation, interactions between the dissolved and the particulate phases in the salinity, redox and turbidity gradients; (Audry et al., 2007; Dabrin et al., 2009; Masson et al., 2011)). Extreme floods, however, may strongly reduce particle residence time, resulting in expulsion of particles that still contain metal fractions which are reactive in estuarine gradients (e.g. salinity). In this case, typical estuarine processes such as Cd desorption by chlorocomplexation (Elbaz-Poulichet et al., 1987) or Cu release by organic matter degradation (Audry et al., 2007; Masson et al., 2011) may occur in the coastal ocean. Particulate trace metal concentrations in SPM from the Gironde and Charente Estuaries were very similar (Table 2; Fig. 3), although for some elements (e.g. Cd, Cu and Pb) there were clear differences for freshwater SPM from both rivers (see above; Table 1, Fig. 3). Comparing metal concentrations in fluvial and estuarine SPM, higher differences occurred in the Charente system, for most of the metals studied (Table 2, Fig. 3). In fact, Cdtot, Cutot and Pbtot were clearly higher in the Charente River than in the estuary, whereas Vtot, Astot and Thtot were lower. In the Gironde Estuary the

A. Dabrin et al. / Continental Shelf Research 72 (2014) 119–130

160

3.5

4

16 12

80

8

Cd (mg kg-1)

120

V/Th

V (mg kg-1)

6.31

3.0

3

2.5 2.0

2 1.5

40

4

1

0

0

0

Cd/Th

126

1.0

140

3.0

120

2.5

100

302.5

60

10

1.0

0.5

0.5

20

00

0

0

Cu (mg

1.5

1

40 5 0

160

25

35 30

Pb (mg

15 10 5

120

25 20

80 15

Pb/Th

kg-1)

20

10

40

5 0

Antioche Strait

WGMP

Gironde sediment

North Bay

South Bay

Charente Estuary

Gironde Estuary

Antioche Strait

WGMP

Gironde sediment

South Bay

North Bay

Charente Estuary

Gironde Estuary

Charente River

Garonne River

Charente River

0

0

Garonne River

Th (mg kg-1)

20 15

2.0

10

25

80

2 1.5

20

0

Cu/Th

3.5

3

kg-1)

3.5 40

As/Th

As (mg kg-1)

0.5

Fig. 3. Averages and standard deviation of total metal concentrations (mg kg
1) (full symbols) and Th-normalised metal concentrations (open symbols) in SPM (squares) and sediment (triangles). The dotted lines separate results obtained for three samples groups: river SPM (Garonne and Charente Rivers; n¼ 23), estuarine/coastal SPM (n¼50) and sediment samples (n¼ 3).

respective values in estuarine SPM were much closer to those in freshwater SPM, although non-conservative behaviour has been reported for Cd, Cu and V, inducing clear concentration changes in the dissolved and, to a lesser extent, in the particulate phases (Dabrin et al., 2009; Masson et al., 2011). These results suggest that either (i) modification of the chemical composition of river-borne SPM during estuarine mixing is more intense in the Charente system than in the Gironde system or (ii) an important part of the Charente Estuary SPM consists of particles having other sources than those actually transported by the Charente River. The total metal concentrations in sediments from the Gironde Estuary and from the WGMP were close to those in SPM from the the Gironde Estuary. Similarly, the total metal concentrations in sediments from the Antioche Strait were close to those in SPM from the Charente Estuary. Assuming that sediments and SPM in the MOB mainly consist of particles derived from the Gironde and Charente Estuaries, the fact that Cdtot and Cutot in SPM from the South part of the Bay were sometimes higher than those in SPM from these estuaries, clearly suggests metal transfer from the dissolved to the particulate phase. This is consistent with previous work on metal dynamics in the oyster production areas of the southern MOB (Pigeot et al., 2006; Strady et al., 2011a). These authors suggest that resuspension of benthic diatoms developing on fine-grained intertidal mudflats (Blanchard and Guarini, 1998), that cover  70% of the MOB, (i) contributes 30–90% to the generally high chlorophyll a levels in the southern MOB (Guarini et al., 2000) and (ii) partly controls metal dynamics. In fact, the

Cdtot in microphytobenthic alga (  1 mg kg
1; Pigeot et al., 2006) were close to those in SPM (  0.88 mg kg
1; this study), supporting that metal accumulation by benthic and/or pelagic algae may recycle metals, by remowing them from the dissolved phase, i.e. involving metal dissolution and stabilisation by estuarine processes (Dabrin, 2009; Masson et al., 2011).

4.3. Assessment of the HCl-available metal fractions in SPM In non-polluted aquatic systems, particulate metals are mainly contained in the so-called residual fraction (i.e. non-reactive fraction; e.g. crystalline lattice), while metals of anthropogenic origin are mostly associated with the reactive fractions of SPM and sediments (Audry et al., 2006). In contrast to the residual fraction, the reactive metal fractions are affected by biogeochemical processes, i.e. potentially bioavailable and of environmental concern. Several operationally-defined speciation approaches exist, in most cases selective extraction procedures using specific chemical agents to extract metals weakly sorbed or bound to labile phases (Sutherland, 2002; Sutherland and Tack, 2008). However, selectivity of the different extractions is limited and bioavailability of metals under environmental conditions depends on many factors (e.g. type of organism, interactions between metals and other dissolved and particulate phases, molecular speciation, nutrient availability, oxygenation of the water column, (Dabrin et al., 2012; Luoma and Rainbow, 2005)).

A. Dabrin et al. / Continental Shelf Research 72 (2014) 119–130

The average HCl-available metal fractions (metalHCl/metaltot) in SPM and sediment from the study area ranged from 90% to 1% according to the following general order Cd¼ Pb 4Cu 4As 4 V 4Th (Fig. 4). This suggests that V, Th and As are mostly bound to the crystalline matrix, i.e. of lithogenic origin, whereas the high reactivity of Cd, Pb and Cu is consistent with previous work in the Garonne River (Audry et al., 2006) supporting both their high potential environmental impact and the important anthropogenic component as reported in previous work (Masson et al., 2006). The HCl-available CdHCl, PbHCl and CuHCl fractions were clearly higher in the Gironde Estuary than in the Charente Estuary suggesting that Gironde Estuary SPM have a higher metal release potential under changing environmental conditions. The CdHCl, CuHCl and PbHCl fractions in the southern MOB (South Bay site) were very similar to those in the Gironde Estuary and much higher than those in the Charente Estuary (Fig. 4). This gradient in the southern MOB may be attributed to (i) dominant inputs of Gironde Estuary SPM via the Maumusson Strait and/or (ii) local accumulation of dissolved metals by benthic algae and plancton (Bassoullet et al., 2000; Guarini et al., 2000; Pigeot et al., 2006). 4.4. Geochemical signals of the residual fraction: a tool to determine SPM sources in coastal environments

100

100

80

80

60 40

60 40 20 0 100

80

80

CuHCl/CuTotal(%)

0 100

60 40 20

60 40 20

0

0

100

100

80

80

PbHCl/PbTotal(%)

60 40 20 0

60 40 20

Antioche Strait

WGMP

Gironde sediment

South Bay

North Bay

Charente Charente

Gironde Estuary

Charente River

Antioche Stait

WGMP

Gironde sediment

South Bay

North Bay

Charente Estuary

Gironde Estuary

Charente River

Garonne River

0 Garonne River

AsHCl/AsTotal(%)

20

ThHCl/ThTotal(%)

systems (e.g. estuaries and coastal zones). Accordingly, the element signature in the contained lithogenic mineral phases is stable even under changing environmental conditions during the transport from the watershed to the ocean which may strongly alter total element concentrations of SPM. Therefore, the residual fraction may carry unique information on particle origin. We have explored for the first time the potential of tracing particle origin and mixing proportions in coastal SPM and sediments from the MOB by comparing metal signatures in the residual fractions of particles from both the Charente and the Garonne Rivers, the latter being the main particle vector into the Gironde Estuary (Schäfer et al., 2002). In fact, the Charente River is the main direct SPM pathway into the MOB (Boutier et al., 2000) and the northward deviation of the turbid plume from the Gironde Estuary is the main plausible indirect SPM tributary to the MOB (Jouanneau et al., 1989; Lesueur and Tastet, 1994). The residual metal concentrations (Vres, Cures, Asres, Cdres, Pbres and Thres) inferred from the difference between total and HCl-available metal concentrations in Charente and Garonne River SPM, were clearly different during the whole hydrological year (Fig. 5). In fact, Vres, Asres and Thres in Charente River SPM were higher than those in Garonne River SPM whatever the hydrological situation, whereas Cdres, Cures and Pbres were lower for most hydrological situations (Fig. 5). Grouping the Th-normalised values of Vres and Asres on the one hand and Cdres, Cures and Pbres on the other, provided two distinct watershed-specific signatures for all hydrological situations. The plot of (Vres+Asres)/Thres over (Cdres+Cures+Pbres)/Thres shows two

CdHCl/CdTotal(%)

VHCl/VTotal(%)

The residual (non-reactive) trace metal fraction does not undergo biogeochemical processes, i.e. it is invariant under variable pH, salinity and redox conditions as occurring in aquatic

127

Fig. 4. Contribution of the HCl-extracted fraction to the total metal concentration (mean and standard deviations) in SPM and sediments from the Garonne and Charente Rivers, and the adjacent coastal systems.

A. Dabrin et al. / Continental Shelf Research 72 (2014) 119–130

16

140

14

120

Sediment

Garonne River

June 2007

Charente River 12

80 60 40

Gironde WGMP

Charente River signature

8

Gironde Estuary

6

Garonne River signature 4

20

Antioche Strait

10

SPM

100

(Vres+Asres)/Thres

Residual particulate concentration (mg kg-1)

128

Ronce les bains La Cayenne Northof the Bay Charente Estuary

2 0

0

V

As

Th

Cd*10

Pb

Cu

Fig. 5. Average residual metal concentrations (mg kg
1) and standard deviations in SPM from the Garonne (○) and Charente (●) Rivers.

well-separated rectangular domains representing the distinct signatures of the Garonne and Charente River SPM obtained from computing the average residual concentrations and their respective relative standard deviations (Fig. 6). Accordingly, the signatures of particles originating from one of these watersheds are expected to plot within the respective domain. In fact, the respective datapoints obtained for SPM from the Gironde Estuary sampled during low water discharge (July 2005) and during a flood event (March 2007) both plot in the Garonne River domain. This result is consistent with the Garonne River being the dominant pathway for particles transported into the Gironde Estuary (Masson et al., 2006), although one cannot exclude that the Dordogne River SPM may have a similar signature. The fact that trace element signatures in SPM from the Gironde Estuary were similar to those in SPM from the Garonne River clearly suggests that (i) the dominance of the Garonne River SPM transport masks potential signals specific to the Dordogne watershed and/or (ii) potential differences in the signals from both watersheds were negligible at the estuary scale. The signatures of surface sediment from the Gironde Estuary and from the WGMP fit exactly with the Garonne signal, corroborating previous sedimentological studies stating that the sediments from the WGMP are mainly derived frome the Gironde system (Jouanneau et al., 1989; Lesueur and Tastet, 1994). Similarly, surface sediment from the Antioche Strait also carry the Garonne River signature, which also fits with previous work reporting possible transport of water masses from the Gironde Estuary via the Antioche Straits driven by high discharge and Southern winds (Boutier et al., 2000; Hermida et al., 1998). The signatures of SPM from the MOB (Ronce les Bains, La Cayenne and North of the Bay sites) and even those sampled within the turbid Charente Estuary salinity gradient plot within or close to the Garonne River signature domain (Fig. 6). These original and unexpected results clearly suggest that in these areas SPM mainly consist of Gironde-derived particles. 4.5. Estimate of the Gironde contribution in the annual SPM budget of Marennes Oleron Bay Based on the distribution of the signature plots obtained for SPM from the MOB, a simple calculation of mixing allowed estimating the proportion of Gironde-derived particles in SPM from the MOB. For this, we used the (Cdres+Cures+Pbres)/Thres values, because this parameter provided more efficient discrimination than the (Vres+Asres)/Thres values (Fig. 6). Assuming that (i) marennes Oleron Bay is mainly influenced by the SPM from the Garonne (Gironde) an the Charentes watersheds, we estimated the mixing proportions for SPM from the Marrenes Oléron Bay (or nay

0

5

10

15

20

25

(Cdres+Cures+Pbres)/Thres

Fig. 6. Plot of (Vres+Asres)/Thres versus (Cdres+Cures+Pbres)/Thres in SPM from the Garonne and Charente Rivers and in SPM/sediments from the costal zone. The rectangular fields representing the Garonne and Charente Rivers signatures were obtained from the average values and standard deviations of both ratios measured in river SPM.

other SPM or sediment) from the respective Garonne and Charente endmember signatures:     Cdres þ Cures þ Pbres Cdres þ Cures þ Pbres ¼X Thres Thres SPM Garome   Cdres þ Cures þ Pbres þð1
XÞ Thres Charente With SPM, corresponding to the signature of any sample of SPM or sediment, X being relative contribution of particles with Garonne signature and 1
X being the percentage relative contribution of particles with Charente signature. For example, SPM sampled at the La Cayenne site in June 2007, i.e. during low discharge in the Gironde Estuary, showed the strongest Charente River component of all MOB samples (Fig. 6) but still contained  54% of Gironde-derived particles. In February 2007, i.e. during high water discharge in the Gironde Estuary (Dabrin et al., 2009), SPM from the same site consisted almost totally of Gironde-derived particles. Our results suggest that even in the Charente Estuary, the proportion of Gironde-derived particles is high (85–93%), which implies low particle transport in the Charente River. Given that SPM and surface sediments in the MOB are frequently mixed due to constant resuspension by strong tidal currents and winds (Bassoullet et al., 2000), we suggest that like SPM, surface sediments contain at least 50% of Gironde-derived particles. During low discharge periods the particle transport via the Gironde Estuary is at minimum, i.e. SPM composition in the MOB probably is very close to that of surface sediment. The signatures of SPM from the North of the Bay sampled during low discharge (July 2005) suggest that the contribution of Gironde particles accounted for  84%. This result is in excellent agreement with previous work using Sr and Nd isotopes, which reported that muddy surface sediments in some areas of the MOB were composed by 40–90% of Gironde-derived particles (Parra et al., 1998). Annual SPM fluxes in the Charente River were 83,000 t in 2006 and 69,000 t in 2007 (Chaniers site). Assuming that all the SPM were transported through the Charente Estuary into the MOB, and that the average contribution (84%) of the Gironde Estuary to particle inputs into the MOB has not changed during the last decades, we suggest that average annual SPM inputs from the Gironde system range between 362,000 t and 435,000 t. Using the minimum contribution of the Gironde Estuary (54%, June 2007 SPM; La Cayenne site) suggests that minimum annual SPM inputs from the Gironde system range between 81,000 t and 97,000 t. These minimum and average contributions respectively correspond to 5–29% of the average SPM

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exportation by the Gironde Estuary (Jouanneau and Latouche, 1982). Accordingly, we estimate average SPM inputs into the MOB via the Gironde and Charente fluvial-estuarine systems at 430,000– 520,000 t y
1, which is to our knowledge the first estimation of annual SPM inputs into the MOB.

5. Conclusions (1) The fact that Cd, Cu and Pb concentrations in SPM from the Charente River were higher than the respective values in the Garonne River, known as contaminated by these metals, raises concern about water quality in the Charente system. Further work is needed to (i) identify sources in the watershed, (ii) quantify metal fluxes and (iii) assess ecological risks as a support for deciders involved in remediation and/or sustainable management of this highly vulnerable zone. (2) Our results suggest that SPM fingerprinting using residual metal concentrations inferred from the total and HCl-extracted fraction is an efficient tool to investigate sources and transport of SPM and sediments from their continental source to the coastal areas. In fact, the non-reactive fraction carries widely invariable geochemical signatures that are conservative even in strong geochemical gradients as occuring in coastal systems. They provide reliable estimates of particle mixing proportions in dynamic coastal receiving particles from different watersheds with distinct signatures. Combining these estimates with SPM fluxes in tributary watersheds may help establishing SPM budgets and impove our understanding on particle and contaminant dynamics in dynamic coastal systems with multiple entries. (3) Although the efficiency of this new tool is evident for the studied system, application to other marine and freshwater environments will provide additional insights in both robustness and need for adaptation to system-specific features.

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